get your wife to try both women specific and small mens bikes , in general women have a longer leg length than men for their given height but a shorter torso therefore women specific bikes usually have a shorter top tube length ( as well as shorter cranks ,softer forks,thinner grips ect).
for what it's worth my wife has a 31" inside leg the same as me but is 5" shorter at 5'5" , she rides a mens extra small Cannondale.

go and do some fittings and test rides, while we do have different body shapes in general to you guys, we have just as much variety between us.

some of the features of womens specific bikes, like thinner grips and shorter reach brake levers are a welcome and much needed feature, but general bike shape should be more about what fits.

Lots of manufacturers have a differnt spin on it too, I ride an XS Cannondale femine frame and it fits me like a glove, but one of my riding buddies cant get on with it at all despite the fact we are almost exactly the same height, she rides a 13inch mens Gary Fisher.

I'd echo carlos' advice - Mrs P tried both ladies and men's (unisex?) models in various different bikes and ended up with a normal Zesty. In her size there was only about 5mm difference in the top tube between the two and the ladies model spec was pretty basic whereas she could go a step up in the normal range.

What is her height as that may affect whether she needs men's or women's?

Hardtail or full suss? You will find girlies on here are quite happy to let other girlies try their bike for size. We can all empathise with lack of demo bikes available in smaller sizes even when we want to spend a shed-load of money on one

I did notice about the sizing though, she's riding a medium womens Myka and i'm on a small (15.5") Mount vision. Although the standover height is more or less identical, riding her bike feels really "sit up & beg" whilst she feels like she is lying flat on my Marin.

My gf also tried various womens and mens bikes, ended up getting a blokes Zesty. Most of the women specific bikes were much short for her as she was used to riding small unisex Kona Hardtails, she is of average proportions.

She asked for advice on the sizing of the womens Zesty, but when she started quoting top tube lengths off the top of her head the dealer admitted she was more clued up than they were so she made her own decision

mrs antigee used to moan about her back when riding until got a wsd trek
and loved it she has (i think writing carefully) relatively short arms and though only slightly shorter than me i find her bike very painful to ride even for a short distance - agree less bargains around in wsd bikes though

It really is about what suits them. Mrs BC currently has a small men's frame whereas my daughter really liked the Trek WSD. Personally when I ride a WSD bike they just feel all wrong and I wouldn't want to be cycling up a steep incline or having to hop up a step on one but perhaps they feel different to the ladies riding them. But you will need to do more than just spin round the car park to try it out.

I am only 5 foot and would only consider a wsd model, small man's just did not have the standover. Poor standover can really knock confidence. If I was taller I would consider a men's. That would mean loads more choice and less flowers!
Really hard to find a tiny 13/14 size in bike shops had to drive miles to get my first bike. How about a demo day? Can book back to back appointments on different sizes, I went to one in glentress a couple of years ago to look at full suz. beats a lap round the car park!

The thing i don't get is for a grand you'll get a decent mens hardtail but an average wsd hardtail.

Examples? I can't think of a single female specific bike currently on sale which is not either a version of the equivalent men's model with appropriate shape-related tweaks to frame/finishing kit, or which sits between two similar men's models in a range but has a slightly different component spec to both and an RRP which reflects that.

Womens bikes should also have a softer fork spring / air fork

No, bikes for lighter riders should use the appropriate spring weight, regardless of gender or spring method.

Yeah, would just repeat what others have said about bikes varying a lot and people varying even more.

I demoed a men's and women's and went for the men's in the end. TT on the Five Diva was a LOT shorter than the Five Pro. However, interestingly the Trek EX8 WSD was identical in geometry to the men's version purely different colour and saddle and grips.

Shorter cranks have been one of the changes I've made and have really appreciated.

Oh and btw, I know why some of you saying don't spend too much, but I'm not sure that's good advice. A heavy bike (especially with weighty wheels) and a poor fork is enough to put anyone who isn't really fit off riding. It was only when I started riding my hubby's bike that I realised how fun mountain biking was and I'm very glad he talked me into trippling our budget for my first 'proper' mountain bike (an 853 Rock Lobster) it was sooooo much easier and more fun to ride than my budget Giant...

Anyway, I just thought I'd even up the arguments a bit. At the end of the day it's up to you and wife but don't be so desperate to not spend much that you don't give her a decent attempt at liking it. Demo days or demos through your lbs are well worth the time and hassle.